Conventionally, in an orthogonal polarization coupler, there is disclosed the one having: a circular main waveguide that transmits orthogonally polarized waves; a coupling hole which is radially provided in order to branch the circular main waveguide; a rectangular sub waveguide that extracts a vertical component electromagnetic wave of the orthogonally polarized waves in the orthogonal direction of the circular main waveguide via the coupling hole; a rectangular sub waveguide that extracts a horizontal component electromagnetic wave of the orthogonally polarized waves in the coaxial direction of the circular main waveguide; a step conversion part for matching the coaxial rectangular sub waveguide with the circular main waveguide; and a septum plate (short circuit plate) that is provided parallel to the horizontal component of the orthogonal polarized waves, and formed in the circular main waveguide on a side closer to the coaxial rectangular sub waveguide with respect to the coupling hole of the circular main waveguide, or a septum plate (short circuit plate) that is provided parallel to the horizontal component of the orthogonal polarized waves, and formed in the step conversion part (e.g., see Patent Documents 1 to 3).
In the orthogonal polarization coupler described in Patent Documents 1 to 3, the orthogonal polarized waves transmitted through the circular main waveguide are branched in the coaxial direction and the orthogonal direction by the septum plate. The polarized wave component parallel to the septum plate is reflected by the septum plate, and extracted in the orthogonally branched rectangular sub waveguide via the coupling hole. On the other hand, the polarized wave of the vertical component orthogonal to the septum plate is extracted from the coaxial rectangular sub waveguide via the step conversion part without receiving much influence of the septum plate. At this time, the step conversion part performs mode conversion from the mode of the circular main waveguide to the mode of the rectangular sub waveguide.
In such an orthogonal polarization coupler, when the polarized wave whose component is orthogonal to the septum plate is extracted, a part of radio waves is reflected on the end of the septum plate, and a part of the reflected radio waves is further reflected on the end of the septum plate on the reversed side. Then, these waves that are subjected to multiple reflection at a certain frequency sometimes overlap and intensify each other, and confine these energies in the section of the septum plate. In such a case, as a result, the radio waves extracted from the rectangular waveguide causes periodic resonance called plate resonance. The frequency at which this periodic and plate resonance occurs depends on the length of the septum plate in the coaxial direction. Therefore, in the orthogonal polarization coupler, in order to effectively extract energy in a desired band, it is necessary to adjust the length of the septum plate.